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      NodeJS学习笔记(三)模块
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        <p>模块（Module）和包（Package）是 Node.js最重要的支柱。开发一个具有一定规模的程序不可能只用一个文件，通常需要把各个功能拆分、封装，然后组合起来，模块正是为了实现这种方式而诞生的。在浏览器 JavaScript 中，脚本模块的拆分和组合通常使用 HTML 的script 标签来实现。 Node.js 提供了 require 函数来调用其他模块，而且模块都是基于文件的，机制十分简单。<br><a id="more"></a></p>
<h2 id="什么是模块？"><a href="#什么是模块？" class="headerlink" title="什么是模块？"></a>什么是模块？</h2><p>模块是 Node.js 应用程序的基本组成部分，文件和模块是一一对应的。换言之，一个Node.js 文件就是一个模块，这个文件可能是 JavaScript 代码、 JSON 或者编译过的 C/C++ 扩展。下面就是我们调用一个模块的例子：<br><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">var</span> http = <span class="built_in">require</span>(<span class="string">'http'</span>);</div></pre></td></tr></table></figure></p>
<p>其中 http是 Node.js的一个核心模块，其内部是用C++实现的，外部用JavaScript封装。只是需要通过require函数获取这个模块，然后才有使用其中的对象。</p>
<h2 id="创建模块"><a href="#创建模块" class="headerlink" title="创建模块"></a>创建模块</h2><p>在 Node.js 中，创建一个模块非常简单，因为一个文件就是一个模块，我们要关注的问题仅仅在于如何在其他文件中获取这个模块。 Node.js 提供了 exports 和 require 两个对象，其中 exports 是模块公开的接口， require 用于从外部获取一个模块的接口，即所获取模块的 exports 对象。<br>首先，创建一个module.js文件，内容如下：<br><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line"><span class="comment">//module.js</span></div><div class="line"><span class="keyword">var</span> name;</div><div class="line">exports.setName = <span class="function"><span class="keyword">function</span>(<span class="params">thyName</span>)</span>&#123;</div><div class="line">    name = thyName;</div><div class="line">&#125;;</div><div class="line">exports.sayHello = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</div><div class="line">    <span class="built_in">console</span>.log(<span class="string">'hello'</span> + name);</div><div class="line">&#125;;</div></pre></td></tr></table></figure></p>
<p>然后在同一目录下创建一个getmodule.js文件，内容如下：<br><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div></pre></td><td class="code"><pre><div class="line"><span class="comment">// getmodule.js</span></div><div class="line"><span class="keyword">var</span> myModules = <span class="built_in">require</span>(<span class="string">'./module'</span>);</div><div class="line">myModules.setName(<span class="string">'心情'</span>);</div><div class="line">myModules.sayHello();</div></pre></td></tr></table></figure></p>
<p>最后运行的结果是<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">hello心情</div></pre></td></tr></table></figure></p>
<p>在以上示例中， module.js 通过 exports 对象把 setName 和 sayHello 作为模块的访问接口，在 getmodule.js 中通过 require(‘./module’) 加载这个模块，然后就可以直接访问 module.js 中 exports 对象的成员函数了。<br>现在，我将上面的module.js文件内容修改一下，内容如下：<br><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div><div class="line">14</div><div class="line">15</div><div class="line">16</div><div class="line">17</div></pre></td><td class="code"><pre><div class="line"><span class="comment">// module.js</span></div><div class="line"><span class="function"><span class="keyword">function</span> <span class="title">hello</span>(<span class="params"></span>)</span>&#123;</div><div class="line">    <span class="keyword">var</span> name;</div><div class="line">    <span class="keyword">this</span>.setName = <span class="function"><span class="keyword">function</span>(<span class="params">thyName</span>)</span>&#123;</div><div class="line">        name = thyName;</div><div class="line">    &#125;;</div><div class="line">    <span class="keyword">this</span>.sayHello = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</div><div class="line">        <span class="built_in">console</span>.log(<span class="string">'hello'</span> + name);</div><div class="line">    &#125;;</div><div class="line">&#125;;</div><div class="line"><span class="built_in">module</span>.exports = hello;</div><div class="line"></div><div class="line"><span class="comment">// getmodule.js</span></div><div class="line"><span class="keyword">var</span> hello = <span class="built_in">require</span>(<span class="string">'./module.js'</span>);</div><div class="line">_hello = <span class="keyword">new</span> hello();</div><div class="line">_hello.setName(<span class="string">'心情'</span>);</div><div class="line">_hello.sayHello();</div></pre></td></tr></table></figure></p>
<p>最后输出的结果是一样的。事实上， exports 本身仅仅是一个普通的空对象，即 {}，它专门用来声明接口，本质上是通过它为模块闭包①的内部建立了一个有限的访问接口。因为它没有任何特殊的地方，所以可以用其他东西来代替，譬如我们上面例子中的 Hello 对象。</p>
<h2 id="警告："><a href="#警告：" class="headerlink" title="警告："></a>警告：</h2><p>不可以通过对 exports 直接赋值代替对 module.exports 赋值。exports 实际上只是一个和 module.exports 指向同一个对象的变量，它本身会在模块执行结束后释放，但 module 不会，因此只能通过指定module.exports 来改变访问接口。</p>

      
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